1. Field of the Invention
The present invention relates generally to rheometers, which are used to characterize materials by measuring the materials' viscosity, elasticity, shear thinning, yield stress, compliance and/or other material properties.
2. Background of the Invention
Rotary rheometers, viscometers or viscosimeters are used to measure fluid or other properties of materials such as their viscosity by rotating, deflecting or oscillating a measuring object in a material, and measuring, for example, the torque required to rotate or deflect or oscillate the object within the material. As used herein, the term “rheometer” shall mean rheometers, viscometers, viscosimeters and similar instruments that are used to measure the properties of fluid or similar (see list below) materials.
The term “measuring object” shall mean an object having any one of several geometries, including, for example, cones, discs, vanes, parallel plates, concentric cylinders and double concentric cylinders. The materials may be liquids, oils, dispersions, suspensions, emulsions, adhesives, biological fluids such as blood, polymers, gels, pastes, slurries, melts, resins, powders or mixtures thereof. Such materials shall all be referred to generically as “fluids” herein. More specific examples of materials include asphalt, chocolate, drilling mud, lubricants, oils, greases, photoresists, liquid cements, elastomers, thermoplastics, thermosets and coatings.
As is known to one of ordinary skill in the art, many different geometries may be used for the measuring object in addition to the cylinders, cones, vanes and plates listed above. The measuring objects may be made of, for example, stainless steel, anodized aluminum or titanium. U.S. Pat. No. 5,777,212 to Sekiguchi et al., U.S. Pat. No. 4,878,377 to Abel and U.S. Pat. No. 4,630,468 to Sweet describe various configurations, constructions and applications of rheometers.
FIG. 1A is a schematic perspective view of a prior art rotary rheometer 100, showing lead screw 101, draw rod 102, optical encoder 103, air bearing 104, drive shaft 105, drag cup motor 106, measuring object 107 (shown in FIG. 1A as a parallel plate), heating/cooling assembly (e.g., a Peltier plate) 108, temperature sensor 110 (e.g., a Pt temperature sensor), surface 111, normal force transducer 112, and auto gap set motor and encoder 113. FIG. 1B is a schematic drawing of a concentric cylinder configuration in position on the rheometer of FIG. 1A, showing the control jacket 120 of the concentric cylinder configuration on top of normal force transducer 112 of rheometer 100. FIG. 1B shows a cylindrical measuring object 121 (used in this configuration instead of the parallel plate measuring object 107 shown in FIG. 1A.
Typical rheometers include essentially two types of bearings for maintaining the position of the shaft, radial bearings and thrust bearings. Modern rheometers utilize air (or other mechanical) bearings for both the thrust and radial bearings because they are non-contact and low friction. The viscosity of high-pressure air in the bearing is one of the limiting factors to the lowest torques that may be applied by the motor, while still resulting in accurate data. One such alternative would be to use a bearing that levitates magnetically.
In rheometers, magnetic bearings have not been fully commercialized. One magenetic bearing that has been utilized in rheometer applications was described by Don Plazek in 1968 (“Magnetic Bearing Torsional Creep Apparatus,” Journal of Polymer Science, A2 6:621–638). This magnetic bearing utilized a combination thrust and radial bearing of a cone and ring shape. Such a magnetic bearing has alignment and preferential position issues and its design is not considered robust enough for typical laboratory use. In addition, this rheometer did not provide the full spectrum of capabilities of typical modern rheometers in that it could only be used to measure creep and was not suitable for other applications such as, for example, steady shear, dynamic and stress relaxation.